This OER eTextbook, An Introduction to Geology, is designed to give you a comprehensive introduction to Geology at no or very nominal cost. It contains both written and graphic text material, intra-text links to other internal material which may aid in understanding topics and concepts, intra-text links to the appendices and glossary for tables and definitions of words, and extra-text links to videos and web material that clarifies and augments topics and concepts. Like any new or scientific subject, Geology has its own vocabulary for geological concepts. For you to converse effectively with this text and colleagues in this earth science course, you will use the language of geology, so comprehending these terms is important. Use the intra-text links to the Glossary and other related material freely to gain familiarity with this language. You can access the OER eTextbook here: http://opengeology.org/textbook/